1. Field of the Invention
The present invention provides a lance for injecting preheated gas into a vessel.
The invention has particular, but not exclusive, application to a lance for injecting a flow of preheated gas into a metallurgical vessel under high temperature conditions.
The metallurgical vessel may for example be a direct smelting vessel in which molten metal is produced by a direct smelting process.
The present invention also provides a direct smelting apparatus which includes a lance for injecting gas into a direct smelting vessel.
2. Description of Related Art
In general, molten bath-based processes for direct smelting ferrous material into molten iron that are described in the prior art require post-combustion of reaction products such as CO and H2 released from a molten bath in order to generate sufficient heat to maintain the temperature of the molten bath.
The prior art generally proposes that post combustion be achieved by injecting oxygen-containing gas via lances that extend into a top space of a direct smelting vessel.
For economic reasons, it is desirable that direct smelting campaigns be relatively long, typically at least one year, and therefore it is important that gas injection lances be capable of withstanding the high temperature environment, typically of the order of 2000xc2x0 C., within the top space of a direct smelting vessel for the prolonged periods of campaigns.
One option for providing oxygen-containing gas is to use air or oxygen-enriched air that is preheated to above 800xc2x0 C.
Stoves or pebble heaters are the only currently viable options for pre-heating air or oxygen-enriched air. One consequence of the use of stoves and pebble heaters is that the air or oxygen-enriched air will pick up hard particulate material as it passes through the stoves and pebble heaters and this material can cause considerable wear to the internal surface of a lance.
The use of air or oxygen-enriched air also means that considerably larger volumes of gas are required to achieve a given level of post combustion than would be required if oxygen was used as the oxygen-containing gas. Consequently, a direct smelting vessel operating with air or oxygen-enriched air must be a considerably larger structure than a direct smelting vessel operating with oxygen.
Consequently, a lance for injecting air or oxygen-enriched air into a direct smelting vessel must be a relatively large structure that can extend a relatively substantial distance into a direct smelting vessel and be unsupported over at least a major part of the length of the lance. By way of context, 6 meter diameter HIsmelt vessels proposed by the applicant include lances having an outer diameter of 1.2 m that are of the order of 60 tonnes and extend approximately 10 m into the vessel.
In addition, such a lance must be capable of delivering relatively large volume flow rates of pre-heated air or oxygen-enriched air and withstanding wear of the interior of the lance due to erosive particulate material in the air or oxygen-enriched air over prolonged smelting campaigns.
For economic and structural reasons, carbon steel is the preferred material for constructing a lance for injecting pre-heated air or oxygen-enriched air.
However, carbon steel is not a preferred material in terms of resisting wear of the interior of the lance and particularly in light of the risk of rapid oxidation (ie ignition) of steel under hot injection conditions.
It is evident from the above that the use of pre-heated air or oxygen-enriched air presents significant issues in terms of the construction of lances for injecting the air or oxygen-enriched air into direct smelting vessels over prolonged smelting campaigns.
An object of the present invention is to provide a water cooled lance that may be constructed using carbon steel as a major structural component of the lance and is capable of injecting pre-heated air or oxygen-enriched air into a direct smelting vessel during a lengthy operating campaign.
According to the present invention there is provided a lance for injecting a pre-heated oxygen-containing gas into a vessel containing a bath of molten material, the lance including:
(a) an elongate gas flow duct extending from a rear to a forward end of the duct from which to discharge gas from the duct, the duct including; (i) inner and outer concentric carbon steel tubes which provide major structural support for the duct, (ii) cooling water supply and return passage means extending through the duct wall from the rear end to the forward end of the duct for supply and return of cooling water to the forward end of the duct, (iii) an exterior surface that includes a mechanical means adapted to hold a layer of frozen slag on the duct;
(b) a gas inlet for introducing hot gas into the rear end of the duct;
(c) tip means joined to the concentric tubes at the forward end of the duct,
(d) a protective lining formed from a refractory or other material that is capable of protecting the duct from exposure to gas flow at 800-1400xc2x0 C. through the duct, the lining being a non-metallic material with heat insulating properties when compared to the steel tubes; and
(e) a means located in the duct for imparting swirl to gas flow through the forward end of the duct.
Preferably the duct includes three or more concentric steel tubes extending to the forward end of the duct.
Preferably the gas inlet includes a refractory body defining a first tubular gas passage aligned with and extending directly to the rear end of the duct and a second tubular gas passage transverse to the first passage to receive hot gas and direct it to the first passage so that the hot gas and any particles entrained therein impinge on the refractory wall of the first passage, with the gas flow undergoing a change of direction in passing from the second passage to the first passage.
Preferably the mechanical means on the exterior surface of the duct includes projections that are shaped to interlock with and hold frozen slag on the duct.
Preferably the projections are lands with each land having an undercut or dovetail cross-section so that the lands are of outwardly diverging formation and serve as keying formations for solidification of slag.
Preferably the tip means is of hollow annular construction and is formed from a copper-containing material.
Preferably the forward end of the duct is formed as a hollow annular tip formation and the duct includes duct tip cooling water supply and return passages for supply of cooling water forwardly along the duct into the tip means and return of that cooling water back along the duct.
Preferably the lance includes an elongate body disposed centrally within the forward end of the duct such that gas flowing through the forward end of the duct flows over and along the elongate central body.
Preferably a forward end of the elongate body and the tip means co-act together and form an annular nozzle for flow of gas from the duct with swirl imparted by the swirl means.
Preferably the swirl means includes a plurality of flow directing vanes connected to the elongate body to impart swirl to gas flow through the forward end of the duct.
In one embodiment of the present invention the elongate body is an elongate central tubular structure extending within the gas flow duct from its rear end to its forward end and the vanes are disposed about the central tubular structure adjacent the forward end of the duct to impart swirl to the gas flow to the forward end of the duct.
Preferably the central tubular structure includes a water cooling passage for flow of cooling water forwardly to its forward end.
More preferably the central tubular structure includes cooling water passages for flow of cooling water forwardly through the central structure from its rear end to its forward end and to internally cool the forward end and thence to return back through the central structure to its rear end.
Preferably the central tubular structure defines a central water flow passage for flow of water forwardly through that structure directly to the forward end of the central structure and an annular water flow passage disposed about the central passage for return flow of water from the forward end of the central structure back to the rear end of that structure.
The central tubular structure may include a central tube providing the central water flow passage and a further tube disposed around the central tube to define said annular water flow passage between the tubes.
Preferably the central tubular structure includes a heat insulating outer shield to retard heat transfer from gas in the gas flow duct into the cooling water passages in the central structure.
The heat insulating shield may include a plurality of tubular segments of heat insulating material disposed end to end to form the heat shield as a substantially continuous tube extending from the rear end to the forward end of the central structure about an annular air gap disposed immediately within the heat shield.
The air gap may be formed between the tubular heat shield and the further tube defining the outer wall of the annular water return flow passage.
Preferably the tubular segments of the heat shield are supported to accommodate longitudinal expansion of each segment independently of the other such segments.
The forward end of the central tubular structure may include a domed nose portion provided internally with a single spiral cooling water passage to receive water from the central water flow passage in the central tubular structure at the tip of the nose and direct that water in a single flow around and backwardly along the nose to cool the nose with a single coherent stream of cooling water.
The central tubular structure may extend centrally through the first gas flow passage of the gas inlet means and rearwardly beyond the gas inlet. The rear end of the central structure may then be located rearwardly of the gas inlet and be provided with water couplings for the flow of cooling water to and from the central tubular structure.
In another, although not the only other, embodiment of the present invention the flow directing vanes are disposed between the elongate central body and the duct to impart swirl to gas flow through the forward end of the duct.
With this embodiment preferably the lance includes:
(a) internal cooling water passage means within the tip means communicating with the cooling water supply and return passage means of the duct so as to receive and return a flow of cooling water to internally cool the duct tip; and
(b) cooling water flow passages within the vanes and the elongate central body and communicating with the cooling water supply and return passage means in the forward end of the duct for flow of water from the supply passage means inwardly through the vanes into the cooling passages of the elongate central body and from those passages outwardly through the vanes to the water return passage means of the duct.
Preferably the cooling water supply and return passage means of the duct include first supply and return passages communicating with the internal cooling water passage means in the tip means and second supply and return passages communicating with the water flow passages in the vanes and the central body.
The tip of the duct may be formed as a hollow annular formation with the hollow formation defining an annular passage constituting the internal cooling water passage means of the tip means.
The carbon steel concentric tubes of the duct may define a series of annular spaces providing the water flow supply and return passage means.
The elongate central body may be generally of cylindrical formation with domed ends.
Preferably the vanes are shaped to a multi-start helical formation. The vanes may then be connected to the duct at multiple locations spaced circumferentially around the duct. Specifically, there may be four vanes arranged in a four start helical formation and connected to the duct at four locations spaced at 90 degree intervals around the duct at the forward ends of the vanes.
The cooling water supply and return passage means of the duct may then include an appropriate number of separated water flow passages each to supply cooling water to one of the vanes. Such separated water flow passages may be formed by dividers within an appropriate annular passage between tubes of the duct extending helically along the duct.
The forward ends of the concentric carbon steel tubes may be connected at their forward ends to the tip means. The rear ends of the tubes may be mounted to allow relative longitudinal movement between them so as to accommodate differential thermal expansion and contraction of the tubes.
The vanes may be connected to the duct and to the central body at their forward ends only so as to be free to move along the duct from those connections under thermal expansion.
The invention also provides an apparatus for producing ferrous metal from a ferrous feed material by a direct smelting process, which apparatus includes a vessel that can contain a bath of molten metal and molten slag and a gas continuous space above the molten bath, which vessel includes:
(a) a hearth formed of refractory material having a base and sides;
(b) side walls extending upwardly from the sides of the hearth, the side walls including water cooled panels;
(c) a means for supplying ferrous feed material and carbonaceous material into the vessel;
(d) a means for generating a gas flow in the molten bath which carries molten material upwardly above a nominal quiescent surface of the molten bath and forms a raised bath;
(e) at least one gas injection lance as described in the preceding paragraphs extending downwardly into the vessel for injecting oxygen-containing gas into the vessel at an angle of 20 to 90xc2x0 relative to a horizontal axis at a velocity of 200-600 m/s and at a temperature of 800-1400xc2x0 C., the lance being located so that:
(i) the lance extends into the vessel a distance that is at least the outer diameter of the forward end of the lance; and
(ii) the forward end of the lance is at least 3 times the outer diameter of the forward end of the lance above a quiescent surface of the molten bath; and
(f) a means for tapping molten metal and slag from the vessel.
Preferably the ferrous feed material and carbonaceous material supply means and the gas flow generating means includes a plurality of lances/tuyeres for injecting ferrous feed material and carbonaceous material with a carrier gas into the molten bath and generating the gas flow.